It is likely that baby is most attentive following his nap. Probably the best indicator as to whether it is a good time to begin interactive stimulation is that your baby is responding to the stimulation. Once he is no longer paying attention, begins to look away, concentrate more on his pacifier, or in any way shows distress (eg crying) it is time to stop.
Now how about you? How are you feeling? Yes, this is just as important! You should always be happy when interacting with your baby. It is okay to skip stimulation activities if you are not up to it. If you are in a bad mood or tired it is actually better to skip the activities for the day. Your baby will still learn many things today, he will still be stimulated, and he will still love you. Take a break and come back tomorrow!